1. Field of the Invention
The present invention relates to an image reading apparatus for optically reading an image such as characters, patterns or the like, or an image such as a coded image represented by a one- or two-dimensional bar code, or the like.
2. Description of the Related Art
Various image reading apparatuses have been known as apparatuses for optically reading an image such as characters, pattern or the like, or an image such as a coded image represented by one- or two-dimensional bar code, or the like.
Some of these apparatuses light a predetermined region including an image to be read by lighting means which have a light source such as an LED, and receive light refracted by the predetermined lighted region, thereby reading the image.
For example, an apparatus disclosed in U.S. Pat. No. 5,724,364 is one of such type.
The image reading apparatus disclosed in U.S. Pat. No. 5,724,364 is configured to detect the maximum value of the image pickup signal supplied from the image pickup means, including image pickup elements such as a CCD, MOS or the like, and perform feedback control on the amount of light generation by the lighting means. More specifically, the apparatus is configured, when the amount of receiving light in image reading is small, to increase the amount of light generation if the light means does not generate light at the maximum. On the other hand, when the amount of receiving light is so high as to exceed the dynamic range of the image pickup element, the apparatus is configured to reduce the amount of light generated if the light means does not generate light at the minimum.
FIG. 1 shows a partial circuit arrangement relating to control of the lighting in the reading apparatus disclosed in U.S. Pat. No. 5,724,364.
When an operator operates a reading switch 1, an operation start signal is input into a CPU 2. In accordance with the input signal, the CPU 2 starts an LED driving circuit 3 so as to cause a LED 4 to emit pulsed light. The start of the emission of the each pulse is synchronized with a perpendicular synchronous signal 6 to be output from an image pickup element 5.
The image pickup element 5 receives the reflected light from a predetermined region lit by the LED 4 and including, for example, an image to be read on paper, through an optical system such as a lens 7 or the like. And then, the element 5 photoelectrically transduces the received light and outputs an image pickup signal, corresponding to the input light, to a maximum value detecting circuit 8.
The maximum value detecting circuit 8 detects the maximum value of one frame and outputs it to the CPU 2. The maximum value detection is renewed for each frame.
The method of the CPU 2 uses to control the amount of light generated will be explained with reference to FIG. 2.
Immediately after the start of image pickup, the CPU 2 causes the LED 4 to emit light at the minimum for performing a proper exposure (Step S1).
Then, the maximum value of the input image at the emitting of the LED 4 is detected (Step S2).
After that, on the basis of the detected maximum value, the optimal exposure time is calculated from a table prepared in advance (Step S3), and the LED 4 is caused to emit light in accordance with the optimal exposure time (Step S4). The control of the amount of light generated by the LED 4 is performed by controlling, for example, the pulse width or pulse height of the generated light.
These operations S2 to S4 are repeated in units of image pickup frames.
With such a structure, it is possible to eliminate variations in the amount of receiving light incident on the image pickup element 5 owing to variations in the LED 4, or variations in operation, such as floating-up of the reading apparatus or the like, and thus it becomes possible to attain an image pickup output having a good S/N ratio and being close to the upper-limit of the dynamic range.
However, in the above-mentioned reading apparatus, when the maximum value of the image pickup signal from the image pickup means is small, the apparatus is configured to emit light at the maximum amount. Therefore, for example, when the reading apparatus is not directed at the image to be read, and instead points midair, no light is reflected back. Thus, the output from the image pickup element 5 becomes very small.
In particular, with respect to what is known as a manually-operated scanning device, such as a reading apparatus which is configured to set an iris small so as to set the depth of field in order to always come into focus on an image pickup plane 9 even though a certain distance exists between the image pickup plane 9 and the apparatus, and is configured to scan and read an object to be read by a manual scanning operation, when it picks up no definite image in such a manner, the incident light is smaller than that of the other type. Thus, an output from the image pickup element 5 can be seldom attained.
As a result, in this case, the LED driving circuit 3 controls the LED 4 to emit light at the maximum in order to attain output from the image pickup element 5.
It can be considered that the output from the image pickup element 5 cannot be attain as, basically, a period where information of an object image to be read is not input. Therefore, it is not preferable to emit light at the maximum at that period since electricity is wasted. Particularly, most portable image reading apparatuses are driven by batteries, and therefore, a problem arises in that the battery life is unnecessarily shortened.